Process for preparing alumina from clays and other silicates of aluminum



vHok-.MAN PROCESS FOR PREPARING ALUMINA FROM CLAYS July 26, 1949.

A AND OTHER SILICATES OF ALUMINUM Filed Aug. 2l,V 1945 /NvE/vv-o/z E/PW/N C. HOE/WAN .....A BY ATTORNEY Patented July 26, 1949 UNITED STATES TENT OFFICE PROCESS- FOR PREPARING ALUMINA FROM CLAYS AND QTHER SILICATES OF ALU- (Granted under the act of March 3, 1883, asy

amended April 30, 1928; 370 0. G. 757) 6; Claims.

The invention described herein may be manufactured and used by or for the Goverment -of the United StatesV for governmental purposes Without the payment to mel of any royalty thereon, in accordance with the provisions of the act of April 30, 1928 (Ch. 460, 45 Stat. L. 467) rlhis invention relates to the manufacture of commercially pure alumina from clays and other silicates of aluminum. More, Particularly, it relates to such a process in which the clay or other silicate is digested by a relatively strong aqueous sulfuric acid solution to produce aluminum sulfate which is converted to alumina by calcination.

The production .of aluminum sulfate by the digestion of aluminum minerals, for example aluminum silicate, in aqueous sulfuric acid is Well known. Similarly, the conversion of aluminum Sulfate to alumina by calclnatiou is also 01d in the art.

The present invention contemplates the production of commercially pure alumina by means of an improved cyclical process involving the treatment of silicates with a strong sulfuric acid solution and thepuriilcation of the resulting aluminum sulfate.

The process oi this invention does not require calcination of the clay or silicate as in the processes of the prior art. The clay or equivalent material is crushed or ground to a suitable size, 4 to 65 meshv Among the objects oi this invention is the provision of a novelmethod for purification of the sulfuric acid solution by removing the metallic impurities, thus providing for the reuse of the acid with accompanying economy in operation.

The invention also provides a new method for the. purification of thc aluminum sulfate crystals resulting from the acid treatment of the Clay, by washing the samevwith strong sulfuric acid S0111- tion, as distinguished from the use of dilute solutions as shown in the prior art.

Still another feature of this process is the provision of a new method for the-conversion of acid aluminum sulfate into normal aluminum sulfate and the recovery of the liberated sulfuric acid and suliur oxides.

Other objects and ieatures of the invention will be apparent to those skilled in the art from description hereinafter given.

Afpreferred embodiment of the invention has been selected for purposes o f illustration and description and is shown in the accompanying drawing, forming a part o f the specification, herein.

The ligure is a iiow sheet illustrating in diagrammatic form the preferred method of carrying out the invention.

Clay or other silicate of aluminum, which need not be calcined, as in most other sulfuric acid processes, is crushed or ground, to a suitable particle sise. The clay is Vdried to the degree required to permit crushing. The process does not require crushing to iinerthan 1/4 inch sieve size. Air dried Clay, Crushed to cass through Sii-mesh, zii-mesh or 4-mesh sieves may be used with equally good results.

The clay is chargedinto a clay digester I, shown diagrammatically in Fig/ 1 ci the accompanying drawing, Where it is digested with a sulfuric acid solution which is preierably maintained at a concentration of about 48"V B.V 'I he digester is supplied with digest acid from filter 5 and with filtered concentrated acid trom acid purification still 2c and filter 2| The ratio of puriiied acid to the remainder oi the digest acid will depend upon the composition of the clay, and vfor al clay containing A12O3, 38.85 per cent; S102 40.22 per cent; FezOs, 3.83 per cent; Tipi, v1.86 per cent; CaO, 0.23

. per cent; MgO, 2.98 per cent; NagO, 0.10 per cent;

KzO, 0.49 per cent; and ignition loss, 8.55, the ratio should be of the order'ot 1 to 9. The quantity of acid supplied is usually about 4 to 5 times the quantity required for reaction with the aluminum content of the clay.

The clay charge is digested with the acid by.

heating in the digester lor a period of from 3 to 5 hours and at a temperature of approximately C. to C., but preferably of 140 C., and may be agitated during heating. Sulfur dioxide gas from the aluminum sulfate Washing system to be described later, is delivered to the digester l. Under these conditions of heating, Water vapor is distilled from the digester, and the acid therein in excess of that required to react with the clay is maintained at a concentration of approximately 48 B, The S02 gas in the digester aids in carrying oli? the water vapor. Alumina extraction from the clay is of the order of 95 to 98 per cent of the alumina present, and the concentration of aluminum sulfate dissolved in the acid will be about 4 to 8 pounds per gallon.

Upon completion of the digestion step the hot pulp is delivered to a residue Washer 2 where it is thickened by settling. The temperature in the washer 2 is maintained by suitable means at 80 C, or higher, so as to retain Ithe aluminum sulfate in solution. The thickened pulp is separated from the clarified acidic liquor and is delivered to residue Washer 3 Where it is washed either with `Water or With acidic Wash liqucr previously used for Washing residue. But it emay be preferred to Wash the residue first with previously used acid Wash liquor of approximately 48 B. concentration, and finally with fresh Water. The Wash liquor after 4 to 6 cycles of use may be of about 25 to 35 C. The crystals are monoclinic in habit and have theapproximate composition Al2(SO4).3H2SO4.7I-I2O; The `slurry cfi crystals formed in crystallizer 4 is delivered to` Vfilter 5 Where they are separated from the digest` acid by filtration and Washed with acid from acid tank 5. crystals is conducted from filter 5 and is re.h1 turned to the clay digester I, the acid purification still to -be more fully set forth in a laterpcrtion of this description, and residue washer 2,;inY

proportions as required ligor maintenance of the balanced operation of the entire system.

Acid tank 6 and filter `5 comprise a portionV of a The system also includes'ta'nks 8,' I 0,12 and lters l',

Washing system for the aluminumA sulfate.

9 and II.V The number of units inthe Washing` system is illustrative and the'operation of the invention is notrestrictedto any fixed number.`

The number of such units may be varied at will to suit the needs of the occasion. Each of the filters 5, 7, 9 and II preferably consists of acontinuous rotary iilter complete with p'ulpervory puddler. In' each lter-puddler unitthe solid aluminum sulfate is puddled with acid Vlfrom the corresponding acid tank E, 8,' YI0 and I2, collected by the rotary filter drum, Washed Ywith additional acid, and finally delivered'to the neXt's'uc-cessive filter unit. Acid for'the last or -nish Washingin Ylter I l is fresh acid produced in the acid recovery system of the process, to be .taken up in greater detail later.

ing system, it is necessary that the Wash acid be saturated with sulfur dioxide to' reduce ferrie sul` fate to `ferrous sulfate; Sulfur dioxideforthis purpose is obtained from the' acid recovery system.

as shown in Fig. 1.

The aluminum Vsulfate.Washing system eliminates the sulfates ofother metals, for example, FeSOi, T1504, CaSO4, MgSQ4,jNaSO4 and K2'SO4,

from the crude aluminum.y sulfate crystallized from the digest acid. Also,'.'in1.the process of Washing in this system, thejinitial .acidic aluminum sulfate having the/approximate composition AI2(SO4)3.SH2SO4.'7H2O .is converted to the less acidic aluminum sulfate havingy the approximate composition Alz(SO-i') 3.H2SO4.3H2O.

The Washed aluminum sulfateisdelivered to a preheater I3 Where it is heate'zd ata temperature of about 140 C..to 180 C. to convertit into sub-` stantially normal aluminum sulfate Alz(SOi)s with a minimum volatilization of sulfuric acid The digest acid separated.fromV the In the operationof the aluminum sulfate washor sulfur oxides. By this treatment the salt will rst dissolve. Thereafter on continued heating and as the temperature reaches 140 to 180 C. the acid will become concentratedand the normal 5 salt Will solidify and precipitate out. A supernatant liquor containing the most of the acid separates and may be removed by dec'antation. The liquid sulfuric Yacid from the decomposition ofthe acid salt [A12 (son amsoifeHgo-Amsoi) 3+ H2SO4+3H201 aluminum sulfateare handled .in a conventionalY system I5, I6 and I'I, in which H2504 is precipitated by electrostatic yprecipitation or other means; ,SO2 is converted toV S03 byV catalytic oxidation` with vanadium vor platinum mass ij; catalyst or other means. The'SOa is absorbed in strong sulfuric acid and hydrated to H2504 which is diluted as requiredto form sulfuric acid of approximately 48. B.. concentration.. A portion of the .SO-ir produced by the decom- `.position of the aluminumsulfateis delivered .to

the acid tanks 6, 8,. I0 and II for use in the Washing of the aluminum. sulfate as previously described and illustrated inr Fig. 1;'r Y

An acid purification still 20 is provided to.

iipurify` preferably about 10v per cent of the acid required for the digestion Aof eachcharge of clay.

Acid is 'supplied to theV still vfrom the preheater I3 and the filter 5.V Acid in this still isV concentrated by evaporation of Water, preferably under re-` duced pressure. 'The treating conditions are adjusted so as to produce asulfuricv acidsolution ofrabout 55 to 60. B..concentration. The concentration of the acid solution v'results vin the precipitation of the metallic sulfates which are relatively soluble in acid solutions of the concen`n trations employed in carrying out thisproc'ess but which are insoluble in acidconcentrations of 55 to B. TheY sulfatevv impurities are'filtered from the puried acid .in filter 2I and may be recovered as by-products ofthe process. The process, as described above and as illus- 55v trated in accompanying Fig.V 1 was operated contiriuously with successive batches of clay. The

analyses of the clay `charges and of the residues from successive'treatments of clay are given inY VVthe following table:

TABLE Analyses of clay and msoluble reszdues Analyses, per cent A1203 Y Extrac- I Y tion A1203 S102 FezOa T102 GaO MgO NaaO KzO Per cent Clay charge--r.- 38. 40. 22 3. 83 1. 86 l 0. 23 2'. 98 0. l0 0. 49 Residue 1 1. 54 53. 52 0.12 0.42 0.15 0.0'5 0.24 0.07 97 02 Residue 2 0.25 0. 80 0. 31 0. 05' 0. 31' '0. 12 93.09 Residue 3 o. 25 0.57 0. 36 0.05 0.24. 0. 08 95. 69 Residue 4 1. 24 1. 54 0. 20 0. 17 0. 05 0. l0 94. 82 1.24 1.40 0.14 0.30 0.09 0.10 97.43 .92 0.50 1.16 0.40 .0.24. 0.16 0.33 96.53 0.05 0.84 0.05 0.05 0.01 0.0i 96.15 y t 0.05 0, 94 0.05 0.05 0.09 0.01 9S. 25 Rcsldue 9 1.79 61.11 0.05 1.51 0.05 0.05, ,0.01, 0.19 96. 97

In the operations, the quantity cf sulfuricacid (48 B.) used for digestion of clay and purification of aluminum sulfate was varied from about 1.75 gallons to 3.5 gallons per pound of clay, and could be kept Within these limits by purifying the acid as described above.

In the operationof the process as described, alumina is obtained having approximately the following composition: A1203, 97 per cent; FezOs, 0.05 per cent; SiOz, 0.03 per cent, TiOz, 0.006 per cent; NazO, 0.05 per cent; and traces of MgO, KzO and H2O.

The process is primarily intended for the recovery of alumina from aluminum silicate and equivalent minerals. However, it may alsobe applied to the recovery of silica and compounds of sodium, potassium, iron, titanium, calcium and magnesium.

The removal of metal salts by digestion of solutions of these salts and aluminum sulfate in sulfuric acid may be made either before or after the treatment of the solution with sulfur dioxide to reduce iron from fer-ric to ferrous state.

To those skilled in the art it Will be apparent that Various modications in the process and ac companying flow sheet can be made Without departingAv from the spirit and scope of the invention as defined in the appended claims.

What I claim is:

1. In the production of aluminum oxide by the conversion of the aluminum content of aluminum silicate containing metallic impurities to acid aluminum sulfate by the action -of strong solutions of sulfuric acid, a method comprising purifying said acid aluminum sulfate by Washing the same with a strong solution of sulfuric acid having a concentration of approximately 48 B. to retain said metallic impurities in solution in the form of sulfates, separating said Washing solution from the purified aluminum sulfate and subsequently removing the same impurities from the solution by concentrating the solution to from 55 to 60 B. so as to precipitate the metallic sulfatos therefrom, and removing the precipitate from the said solution.

2. In the production of aluminum oxide by the conversion of the aluminum content of aluminum silicate containing metallic impurities to acid aluminum sulfate by the action of strong solutions of sulfuric acid, a method comprising purifying said acid aluminum sulfate by Washing the same in the presence of sulfur dioxide with a strong solution of sulfuric acid having a concentration of approximately 48 B. separating said Washing solution from the purified aluminum sulfate and subsequently removing the said impurities from the solution by concentratingthe solution so as to precipitate the metallic sulfates therefrom.

3. In the production of aluminum oxide by the treatment with sulfuric acid of aluminum silicate containing metallic impurities, a cyclic method comprising subjecting crushed air dried clay in a digester to the action of 48 B. solution of sulfuric acid at an elevated temperature for a suiiicient period of time to convert substantially all the aluminum content of the clay to acid alumi num sulfate, separating the digest liquid from the solidresidue by filtration, Washing the residue first with a sulfuric acid solution of approximately 48 B., then With Water, returning the acid Wash solution to the digester, crystallizing acid aluminum sulfate from the digest liquid by cooling, separating the crystals from the digest liquid, by filtration, washing the crystals with 48 B.

sulfuric acid solution in the presence lof sulfur dioxide to remove impurities therefrom, ,conducting the purified crystals to a heating chamber to be heated at a temperature of from 140 t0 180 C. to separate the sulfuric acid and'water ccmponents to thus produce normal aluminum sul fate, separating the liberated sulfuric acid :and Water from the normal sulfate, then calcin'ing the normal aluminum sulfate `to decompose it to aluminum oxide and the oxides of sulfunreturnl ing a portion of the acid solution from the crystal Washing and filtering operations to the digester, and concentrating another portion' of thel acid solution together With acid from the heating chamber to a concentration of to 60 Berto precipitate metallic sulfate impurities therefrom, and reusing the purified acid inthe process, Vrecovering the sulfur oxides and using a portionin the crystal washing operation and a portion in the digester and converting the remainder to sulfuric acid to be reused in the process,

4. In the production of aluminum oxide by the treatment with sulfuric acid of aluminum silicate containing metallic impurities, a cyclic method comprising subjecting crushed air dried clay in 'a digester tothe action of a strong solution of suln` furie acid at an elevated temperature for a sufficient period of time to convert substantially all the aluminum content of the clay to acid aluminum sulfate, separating the digest liquid from the solid residue by filtration, Washing the residue first with a strong sulfuric acid solution, then with Water, returning the acid Wash solution to the digester, crystallizing acid aluminum sulfate from the digest liquid by cooling, separating the crystals from the digest liquid, by filtration, Washing the crystals with a strong sulfuric acid solution in the presence of sulfur dioxide to remove impurities therefrom, conducting the purified crystals to a heating chamber to be heated at a temperature sufficient to separate the sulfuric acid and water components Without causing the formation of oxides, to thus produce normal aluminum sulfate, separating the liberated sulfuric acid and Water from the normal sulfate then calcining the normal aluminum sulfate to decompose it to aluminum oxide and the oxides of sulfur, returning a portion of the acid solution from the crystal Washing and filtering operations to the digester, and concentrating another portion vof the acid solution together with acid from the heating chamber so as to precipitate metallic sulfate impurities therefrom, and reusing the purified acid in the process, recovering the sulfur oxides and-using a portion in the crystal washing operation and a portion in the digester and converting the remainder to sulfuric acid to be reused in the process.

5. A method for purifying and converting acid aluminum sulfate having the approximate composition Al2(SO4)33H2SO4'7I-I2O to normal aluminum sulfate, comprising Washing said sulfate With a concentrated solution of sulfuric acid, separating the Wash acid, and heating the Washed acid sulfate to a temperature of to 180 C. to liberate sulfuric acid and Water and separating the sulfate from the liberated acid and Water.

6. In the production of aluminum oxide by the treatment with sulfuric acid of aluminum silicate containing metallic impurities, a cyclic method comprising subjecting crushed, air dried clay in a digester to the action of a solution of sulfuric acid of approximately 48 B. concentration at an elevated temperature for a suflicient period of time to convert substantially all the aluminum content-of tneiclay tofacid aluminumsulfate lhavling the approximate composition due, washingtheresidue first with a sulfuric acid Y solutionjof approximately 48 B., then with water, returning theacid wash ,solution to4 the digestengcrystallizing the acid aluminum sulfate from kfthe digest liquidby cooling, separating the crystals `fron-1 the digestv liquid, Washing the crystals with a sulfuric acid solution of approximately 48 B, concentrationsin the presence of sulfur dioxide, conducting the washed crystals to a heating'chamber to be heated at a temperature lof fron1l140 1:07189?A C. to form normal aluminum sulfate, separating the liberated sulfuric acid and Waterfrom thenormal sulfate then calcining the normal aluminum sulfate'to decompose it to aluminum oxide and oxides of sulfur, returning a portion of the acid solution from the crystal washing operation to the digester, and concentrating another portionof` said acid solution together withacid .from the heating chamber to a concentration of 55 tov60? B. to precipitate impurities therefrom, and reusing `the purified acid 'in the process, recovering the sulfur oxide and using a -portion in thecrystal Washing operation and 10 i Number y'Name Y' Dateyfv 1,538,837 Moxham 1 May 19, 192,5k 1,607,279v Hltlnail Y ....Q NOV. 16, 1926` 2,249,761 d Hixn '.L;A July 22,1941 15 2,230,508 Bousquet'etai 1; `Apr.21,1942 s i FOREIGN PATENTS .y

Number Country ,Y Date 805,325 France a-; Nov.`17, 1936 20 v l 8 Y a-portiori in 'the `viligester and vconiferting the `re mainder to sulfuric acid to be reused in the proces l Y Y 'Ihe following referenlces areof? recroidjin@theY Y Y' OTHER REFERENCES Tilley et al.:. Acid Processes for the Extraction of Alumina, U. S. Bureau of Mines Bulletin No. 267, pp. 58 and 59 (1927): L y

Mellor.: Inorganic and Theoretical Chemistry, vol. 5, p. 336, Longmans, London (1924i) Gmeun-Kraut, Handbuch vder AnorganischenVY Chemie, vol. 2, part-2,`Heide1berg `(1909) 11164. 

